Extraction of mercury from alkaline brines

ABSTRACT

This invention is for a method of quantitatively removing mercury from aqueous waste streams with certain selected liquid organic amines. The method in its preferred form is particularly appropriate for removing mercury from aqueous brine solutions. Efficient and quantitative recovery of the extracted mercury is also provided.

United States Patent [191 Moore Jan. 15, 1974 EXTRACTION OF MERCURY FROMALKALINE BRINES [75] Inventor: Fletcher L. Moore, Knoxville, Tenn.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission, Washington, DC.

221 Filed: Mar. 1, 1972 21 Appl. No.: 231,029

[52] US. Cl. 75/121, 75/101 [51] Int. Cl C22b 43/00 [58] Field of Search75/101, 121; 423/99 [56] References Cited UNITED STATES PATENTS3,083,085 3/1963 Lewis et al 75/121 3,085,859 4/1963 Scholten et a1.75/121 Primary Examiner-Oscar R. Vertiz Assistant ExaminerS. B. ShearAttorneyJohn A. Horan [57] ABSTRACT 2 Claims, No Drawings BACKGROUND OFTHE INVENTION The invention described herein was made in the course of,or under, a contract with the U. S. Atomic Energy Commission.

The present invention relates to a process for the abatement of mercurypollution. More particularly, it relates to a method for the removal andefficient recovery of mercury from aqueous effluents generated byindustrial processes. A major source of environmental mercury comes fromthe brine effluents issuing from chlor-alkali plants, aqueous effluentsfrom paper and pulp mills, and from mercury ore beneficiation proc-'esses. A listing of mercury dischargers into public waters is found inORNL NSF-EP-l Mercury in the Environment, a publication of the Oak RidgeNational Laboratory, Oak Ridge, Tennessee. This publication documentsthe toxic effects of mercury upon man and his environment and points outthe need for reducing the mercury level in mans total environment.

It is a general object of this invention to respond at least partiallyto this need by providing a process which allows quantitative removaland recovery of mercury from aqueous waste streams of the characterdescribed. A particular object of this invention is to remove andrecover mercury from aqueous brine solutions.

THE PRIOR ART Several methods are available for removing mercury fromaqueous solutions. The precipitation of mercury by a more active metalsuch as aluminum, copper, iron, or zinc is effective, but it substitutesanother metal in solution, usually in considerably more than anequivalent amount. There are methods which are based on theprecipitation of mercuric sulfide, but these suffer from high toxicityand offensive odor of sulfide reagent. Sorption of mercury on preformedmetal sulfides such as iron, cadmium, or zinc sulfides is available,but, as in the precipitation methods, it results in replacing themercury with another metal ion.

Ion exchange methods which may be available are expensive and sufferfrom problems arising from resin attrition and fouling arising fromsuspended solids present in industrial solutions.

A limited class of organic analytical reagents such as dithizone areknown to form organic complexes with mercury, but such reagents areexpensive and'chemically unstable.

Any promising pollution control process for removing mercury" fromaqueous solutions is one characterized by the use of a cheap, relativelyavailable selective reagent for mercury which can quantitatively removemercury from solution. While removal of the mercury is a technicalrequisite, a process which allows quantitative recovery of the mercurywould be one which would most likely be accepted and used by those whodischarge mercury-containing solutions into public streams. it istherefore an object of this invention to provide a process which allowsfor both removal and recovery of mercury from aqueous solutions and fromindustrial waste solutions.

mercury from aqueous solutions can be effected by liquid-liquid solventextraction utilizing an organic phase I containing selectedaqueous-insoluble high-molecularweight organic amines as the selectiveextractants. The invention entails the use of selected liquidaqueousinsoluble primary, secondary, tertiary, and quaternary amineswhich extract at least 99 percent of the mercury from a mercurycontaining aqueous solution.

To be a practical industrial extractant the solvent must extract mercuryfrom an alkaline or basic solution. Of the class of organic amines whicheffectively remove mercury at the 99-percent level, any of the class ofquaternary amines to be described can extract mercury over a wide rangeof pH. On the other hand, the effectiveness of the primary, secondary,and tertiary amines is much more sensitive to the pH of themercury-containing aqueous phase.

The mechanism responsible for the extractability of mercury apparentlydepends on the existence of mercury as an anionic mercuric complex whichcombines with the amine phase to form an aqueous-insoluble amine complexhighly soluble in most organic solvents. The process of this inventionis especially effective for the extraction of mercury from aqueous brinesolutions. In such solutions, the mercury is already present inextractable form as the mercuric chloride complex (HgCl Thus, noaddition of chemicals or solution adjustments are necessary where theextractant is a quaternary amine chloride salt. All that is required isintimate contacting of the mercury-containing brine solution, by eithermixer-settler or countercurrent techniques, with the amine phasedissolved in a hydrocarbon solvent such as xylene, diethylbenzene, orkerosene. Other useful diluents are chlorinated hydrocarbons, alcohols,and ketones.

Where the extractant is a quaternary amine chloride, the reaction can bevisualized as involving:

where:

R NCl a quaternary amine chloride such as tricaprylmethylammoniumchloride; 0 organic phase; and a aqueous phase.

DETAILED DESCRIPTION OF THE INVENTION The quaternary amine chloridesalts are prepared by mixing the free quaternary base with hydrochloricacid. Stock solutions 0.68 M dissolved in xylene and diethylbenzene wereprepared by mixing the solution with equal volumes of l M HCl forseveral minutes. After centrifugation the organic phase was separatedand used in the following examples in the original or dilutedconcentration. In the following examples an aqueous solution was spikedwith mercury-203 tracer. 5 millimeters of the indicated aqueous phasewas extracted at room temperature with equal volumes of the indicatedorganic phase by mixing the two phases using a highspeed motor-drivenstirrer for 3-minute mixing periods. After extraction, theaqueous-organic mixture was centrifuged. Each phase was then analyzedfor mercury- 203 by counting l-rnl aliquots in a well-type gammascintillation counter. Using this general procedure, the followingexamples illustrate a best mode of practicing the invention with atypical quaternary ammonium chloride salt.

EXAMPLE 1 The extraction of mercury-203 with 30 percent weight by volume(W/V) tricapryl monomethyl ammonium chloride as a function ofhydrochloric acid concentration is shown in Table l below. Thedesignation Aliquot 336-S-Cl in the examples refers to the tricaprylmonomethyl ammonium chloride salt available from General Mills, lnc.,Kankakee, Illinois.

TABLE 1 Extraction of "Hg Tracer with 30% Aliquot 336-SCl-Xylene as aFunction of HCI Concentration HCl, M "'Hg Tracer Extracted,

' weight Xv/by volume (e.g., 300 g. reagent per liter for 30% WW) ltwill be seen that at least 99 percent of Hg is extracted over a widerange of hydrochloric acid concentration. The decrease in mercuryextraction at very dilute HCl concentrations is thought to occur becauseof the tendency to form water-soluble hydroxy species of mercury.However, this can be averted either by pretreatment of the extractantwith HCl or by the addition of HCl or NaCl to the aqueous solution.

EXAMPLE ll The effect of sodium chloride concentration is shown in Tablell below.

TABLE ll Extraction of Hg Tracer with 30% Aliquot 336-S-Cl-Xylene as aFunction of NaCl Concentration Initial Aqueous Phase NaCl, M HCl, M mTracer Extracted,

It is seen that excellent recovery, greater than 99 percent, of theHg-203 tracer was achieved over a wide range of sodium concentration inthe aqueous phase. While these results were obtained with mixing periodsof three minutes, l have obtained essentially equivalent results withmixing periods as short as 30 seconds, indicating that the extractionoccurs rapidly.

EXAMPLE III This example shows that macro concentrations of Hg will alsoextract quantitatively from aqueous solutions of hydrochloric acid andsodium chloride. Table lll below summarizes results obtained withaqueous solutions containing various concentrations of mercury, HCl, andsodium chloride.

TABLE III Extraction of Macro Concentrations of Mercury from ChlorideSolutions with 30% Aliquot 336-S-Cl-Xylene lnitial Aqueous Phase NaCl, MHCl, M Hg, mg/ml Hg Extracted,

0.1 8 99.8 0.3 6 99.8 0.3 2 99.8 1.7 2 99.9 6.l 2 99.9 7.3 2 99.3 0.50.95 2 99.7 1.0 0.05 2 99.7 3.0 0.05 2 99.9 3.9 0.02 2 99.9 l.0 O.l0 699.7 3.0 0.04 lS.2 99.9

Further tests have shown that up to milligrams of mercury per millimeterof extractant could be loaded into the organic phase. The aminezmercurymole ratio was about 2:1. The organiczaqueous ratio can be quite low (aslow as 0.06) provided the aminezmercury molar ratio is at least 2.

EXAMPLE lV TABLE IV Extraction of Mercury with 30% Aliquot 336-S'Xylcncas a Function of HNO, Concentration Hg Extracted,

HNO,-,, M R NNO= -R NCl Initial aqueous solutions contained 2 mglml HgIt is seen that mercury extracts fairly well from dilute nitric acidsolutions only and that, at a given nitric acid concentration, Hgextractability is greatly enhanced by use of the chloride salt of theamine.

EXAMPLE V A practical industrial extractant for mercury should functionequally well from alkaline as well as acidic brines. An importantadvantage of the quaternary amine chlorides is their ability toquantitatively extract mercury from aqueous brine solutions having awide range of pH. This is shown in the results for the extraction ofmercury as a function of pH of the brine solution.

TABLE v Extraction of Mercury from Brine Solutions with 30% Aliquot336-S-Cl-Xylene as a Function of pH Aqueous Phase pH Initial Final HgExtracted,

' Initial aqueous solutions contained 3 M NaCl and 2 mglml Hg 1tieiiihiiiir55699 irciifoftlifig'wasix tracted from solutions ranging inpH from as low as 1 to as high as 10.9. The extraction of Hg fromalkaline or acidic brine solutions of potassium chloride was found to beessentially identical to that found in the sodium chloride system. Ihave also achieved quantitative removal of mercury from sodiumhypochlorite solutions.

Previous Examples l-V, inclusive, have been presented as an example of abest or preferred mode of practicing the invention-that is, by using thechloride salt of a quaternary amine. While tricapryl monomethyl ammoniumchloride was used in these examples, it was used to represent thegeneral class of quaternary amines which exhibit exceptional capacity toquantitatively extract mercury from acidic or basic brine solutions. Theclass of effective aqueous insoluble quaternary amines suitable forrealizing the objects of this invention may be represented by thefollowing generic formula:

where R represents a straight-chain or branc hal alltyl or aryl group.At least two of the hydrocarbon groups should contain from six to 18carbons in the chain, the compounds being soluble in such organicsolvents as xylene, diethylbenzene, and other hydrocarbon solvents.Particularly useful are those compounds in which three of the groups aresaturated hydrocarbon chains containing eight to 10 carbon atoms eachand the remaining group is a methyl group. Specific examples ofquaternary ammonium salts are tricaprylmethylammonium chloride,trihexylmethylammonium chloride, trioctylmethylammonium chloride,tridecylmethylammonium chloride, distearyl dimethylammonium chloride,and lauryldimethyl benzylammonium chloride.

It should be understood that other high-molecularweight (i.e., MW200-500) amine salts will be found to be effective in extracting mercuryfrom aqueous solutions. While selected primary, secondary, and tertiaryamines will extract mercury from acid solutions, their performance asselective, efficient mercury extractants at the 99 percent level inextracting mercury from basic solutions is not as good as the quaternarysalts. While there is no definite basis for accounting for the variedextraction power of the primary, secondary, and tertiary amines in basicsolutions, it seems plausible to as sume that the amine salt form is notas stable in basic solutions and reacts to the less effective free baseform.

The extraction of mercury from alkaline brine solutions in short-timecontacts as a function of amine class is shown in Table VI.

'rKBLei/i Extraction of Mercury from Alkaline Brine Solutions as aFunction of Amine Class* Aqueous pH No. Chemical Name Class InitialFinal Hg Extracted,

1 RNH,, where R==mixture of isomers Primary 10.6 9.6 99.9

having 12-14 carbons, with t-alkyl groups" 2 RNH where R=mixture ofisomers Primary 10.5 9.5 99.9

having 18-22 carbons, with t-alkyl groups" 3N-Dodeceny1(trialkylmethyl)-amine, Secondary 10.5 7.4 70.0

trialkylmethyl-alkyl groups contain 12-15 carbons 4N-Lauryl(trialkylmethyl )-amine- Secondary 10.5 9.4 15 .0

alkyl group contain 12-15 carbons 5 Tricaprylamine, NR the R groupsTertiary 10.5 8.6 40.0

are a C C mixture 6 Tricaprylmethylammonium chloride. Quaternary 10.68.7 99.9

NRQCHQCI; R is a mixture of C -C carbons 7 Same as I (T) Primary 10.68.3 99.9 8 Same as 2 (T) Primary 10.5 6.1 54.8 9 Same as 3 (T) Secondary10.6 2.2 99.9 10 Same as 4 (T) Secondary 10.5 4.2 99.9 l 1 Same as 5 (T)Tertiary 10.5 4.2 99.9 12 Same as 6 (T) Quaternary 10.6 1.2 999 Initialaqueous solutions contained 3 M NaCl and 2 rug/ml Hg.

(T)=amin: cxtractant pretreated with l M HCl.

Available as 30% Prirnene 81R from Rohm & Haas, Philadelphia. Pa.Available as 30% Primcne JMT from Rolun '8: Haas, Philadelphia, Pa.Available as 30% Amberlite LA-l from Rohln & Haas Co., Philadelphia, Pa.Available as 30% Amberlite LA-2 from Rohm 8: Haas (30., Philadelphia.Pa. 'Available as 30% Alarnine 3368 from General Mills. Kankakee, Ill.'Available as Aliquot 336-S from General Mills, Kankakee, lll.

Regeneration of Quaternary Amine Solvent For practical processapplication, a satisfactory extractant must be regenerated so that itcan be recycled. The following example illustrates how the mercury canbe recovered from the organic phase.

EXAMPLE V1 (A) Aqueous Strippants A number of aqueous solutions ofinorganic and organic reagents were evaluated for their ability to stripmercury from xylene solutions containing 2 mg/ml mercury. The resultsare summarized in Table VI].

' "rXiiE vii Stripping of Mercury from Tricapryl Monomethyl AmmoniumChloride-Xylene Solution Hg Stripped,

" it will bEriBFedtTi misses firiiifnEdTEHThEf from the organic phasebetter than any other acidic reagent, but required a fairly high nitricacid molarity.

Ammonium hydroxide stripped the mercury effectively, but someprecipitate fonfiedififivr high concentrations.

The best stripping agents were dilute aqueous solutions ofethylenediamine and propylenediamine. Greater than 99 percent of themercury was easily stripped by each of these reagents withoutprecipitation. Other .organic diamines should strip the mercury withsimilar effectiveness.

EXAMPLE V11 (B) Reduction-Precipitation of Mercury with Metal Analternative method for stripping the mercury from the extractant is toprecipitate the mercury directly by reduction with a more active metal.According to this aspect of the invention, such elements as aluminum,iron, copper, and zinc, preferably in finely divided form as turnings oras powder, will convert the mercury in the extractant directly tometallic mercury.

In a typical experiment a volume of 8 ml containing 2 mg/ml mercury wasmixed for 3 minutes with 2.3 grams of aluminum turnings and then allowedto remain in contact an additional time. The results are shown in TableV111.

TABLE V111 Reduction-Precipitation of Mercury from 5% TricaprylMonomcthyl Ammonium Chloride-Diethyl Benzene by Aluminum Reduction Time,Min. Hg Reduced,

90.3 Overnight (18.5 hr.) 98.0

The results show that most of the mercury was removed in 2 hours andessentially quantitative removal occurred after overnight contact.Faster reduction will occur with continuous mixing with aluminum shot orpowder.

EXAMPLE V111 This example is intended to demonstrate extraction andrecovery of mercury from alkaline brine feed solution. The feed solutionhad a pH of 10.6, contained 3 M NaCl, 2 mg/ml mercury, as well as Hgtracer 5 X 10 gamma counts/min/ml. 20 milliliters of the alkaline brinesolution was extracted with 20 ml of 5 percent tricapryl monomethylammonium chloride dissolved in diethylbenzene. The mercury was thenstripped from the solvent by extracting for 3 minutes with 20 ml of anappropriate strippant. The strippants evaluated were 15 M NH OH, 2.5percent ethylenediamine, and 2.5 percent propylenediamine.

The regenerated solvents were then used in a second cycle to extractfresh portions of the alkaline brine solution.

Concentrated ammonium hydroxide (15 M) stripped the mercury essentiallyquantitatively, but often a slight amount of mercury precipitated andmaterial balances were poor. On the other hand, dilute aqueous solutionsof ethylenediamine or propylenediamine quantitatively stripped themercury from the solvent with no evidence of mercury precipitation.Typical results are shown in Table 1X;

TABLE 1X Recovery of Mercury from Alkaline Brine Solutions with 5%Tricapryl Monomethyl Ammonium Chloride-Diethyl Benzene via Two-CycleProcess Aqueous pH Process Step Mercury lnitial Final Recovered,

(Run No. 1) Initial Extraction 99.9 10.6 1.9 2.5% ethylenediaminestrippant 99.6 Second-cycle extraction 99.9 10.6 10.7 Second-cyclestrippant 99.6

(Run No. 2) Initial Extraction 999 10.6 1.9

2.5% propylenediamine strippant 99.8 Second-cycle extraction 99.9 10.610.8 Second-cycle strippant 99.7

The data of Table 1X for two runs using two effective strippants showthe essentially complete removal and recovery of mercury from an aqueousbrine solution. Because of the high extraction coefficient for mercuryin a single stage, multiple extractions may-not be necessary.

To summarize, l have shown that certain quaternary amine salts haveextraordinary capacity to remove mercury from acidic as well as alkalinebrine solutions. The capacity of these reagents to extract mercurysatisfies the dual purpose of abatement of mercury pollution and theremoval of mercury from brine and other aqueous chloride solutions. Andthe quantitative and efficient stripping action of the lower alkyldiamines, ethylenediamine and propylene diamine, allows recycling of theextractant and virtually complete recovery of mercury. The mercury canbe converted to elemental form by destruction distillation of thediamine solution under reduced (subatmospheric) pressure. The process istechnically effective, simple, and efficient and provides an economicincentive to the user since the mercury can be virtually completelyrecovered for reuse. And while the examples are directed to ademonstration of the efficacy of chloride salts, it is within the scopeof this invention to employ nitrate, sulfate, phosphate salt forms wherethe circumstances require. For processing chloride solutions, however,the quaternary amine salts are the most economical and effective.

What is claimed is:

1. In the method of removing and recovering mercury from alkaline oracidic aqueous solution in which said solution is contacted with anorganic phase consisting essentially of a quaternary amine salt and asolvent therefore, said amine salt having the general formula whemprients a straight chain onbr anched chain alkyl or aryl hydrocarbongroup, R is methyl, a straight chain or branched chain alkyl or arylhydrocarbon group, and X represents Cl, or S0,, or N0 or PO, anion,where at least two of the hydrocarbon groups contain from six to 18carbon atoms, under such conditions as to effect transfer of the mercuryinto the organic phase, the improvement which comprises stripping themercury from the organic phase with an aqueous solution of a strippantselected from the group consisting of ethylenediamine andpropylenediamine.

2. The method according to claim 1 where R capryl and R methyl.

2. The method according to claim 1 where R capryl and R1 methyl.